258 ESTIMATION OF STANDING TIMBER 



uring each tree is far too great to be undertaken. It is often physically 

 impossible to obtain the necessary force and personnel to perform the 

 work on this scale. Finally, the time required is too long since the 

 results of estimates, especially for the purpose of sale are usually required 

 within a limited period. For these reasons, the third of the above 

 methods, by which the principle of averages is utilized as a means of 

 reducing expense, diminishing the number of persons required and 

 shortening the time demanded for completing the work, is almost 

 universally used in estimating timber. 



The use of this principle in timber estimating does not differ from 

 that applied in the commercial process of sampling employed in mines 

 or in grad'ng wheat. If the product is uniform, a single sample suffices, 

 as in wheat, but if variable, as in ore, far greater care is required in 

 order that the samples may represent the average value for the entire 

 body to be tested. The advantage in timber estimating is that all 

 of the timber is actually visible and only the handicap of costs and 

 time prevent it from being seen and measured. 



209. The Six Classes of Averages Employed in Timber Estimating. 

 There are six classes of averages employed in estimating timber. The 

 first three differ in regard to the methods of recording the dimensions 

 of trees. These methods are as follows: 



1. The average height of the trees of each separate diameter class 

 is obtained For this purpose, only a few sample heights for each 

 separate diameter are measured. The heights so measured are plotted 

 on cross-section paper on which diameter is the determinate variable 

 plotted on the horizontal scale, while height is the indeterminate vari- 

 able plotted on the vertical scale. 



An illustration of a curve to obtain average heights based on diameter is shown 

 in Fig. 56. The trees to be measured for height must be selected in such a manner 

 that the resultant curve will give the true average heights for each diameter class 

 for the entire area to which it is to be applied. When a very few trees are taken, 

 these must be carefully chosen from those whose crowns are of average height 

 compared with the remaining stand. This is best accomplished in even-aged stands. 

 On large areas and in many-aged stands, a mechanical distribution of trees measured 

 for height is best, in order to secure a weighted average of differences caused by 

 variation of site and of growth. 



In plotting the data, two methods are shown. By the first, all heights are 

 plotted above their respective diameters. A height curve may thus be sketched 

 by eye through the band of points shown. This eliminates mechanical averaging. 

 By the second method, the average height is calculated for the trees in each diameter 

 class, and this point is plotted ® . The points are then connected by straight 

 lines, their weight in numbers shown, and the curve drawn, as before, guided by 

 the original data.^ 



' In the first system, when two heights fall on the same point, the number is 

 indicated as ^. 



